The present invention relates to microlithographic systems of the type used for the manufacture of semiconductor devices by the printing of successive circuit patterns on the surface of a semiconductor wafer, and more particularly, to such a system in which alignment between successive exposures is provided by means of a microscope mounted to one side of the optical projection column.
As is understood by those skilled in the art, the successful manufacturer of complex semiconductor devices requires that very precise registration be maintained between the successive exposures which are used to produce the successive layers required in building up such devices. To provide a basis establishing registration between successive exposures, alignment systems typically employ a system of fiducial markings. These markings start out on a reticle or mask of the type used for forming the semiconductor circuits themselves and are transferred to the surface of a wafer by the lithographic process. After development of the first exposure the mark is observed by means of a microscope and is used to initially position each wafer prior to its being moved into the projections column for subsequent exposures.
One of the problems with this prior art system is that it relies on there being a precisely defined separation or base line vector between the point of observation and the point in the optical projection column which corresponds to the original location of the fiducial mark on the wafer surface. To establish this base line vector, it has been typically necessary to employ an elaborate setup or initialization procedure in which two successive reticles are used to expose the wafer and, after respective developments, the accuracy of the registration was determined by microscopic observation. Typically, the two successive reticles provided patterns which included x-and-y-axis vernier scales to facilitate quantitative measurement of any offset.
Not only is this procedure time consuming and therefore expensive, involving as it does two successive development procedures before any feedback information is obtained, but the end result is a function not only of the base line vector but also of the accuracy with which the second reticle was aligned with the first and also the degree of accuracy with which the wafer was initially positioned prior to its second trip into the optical column for exposure. Further, the time required for this procedure is so long, i.e., in the order of twenty five minutes or greater, that the base line vector may in fact drift during this period. As is understood by those skilled in the art, baseline drift can arise through a variety of causes including such innocuous effects as slight thermal changes in the optical column itself, barometric changes or mechanical shock. While these changes may be small in an everyday context, they can have considerable significance when it is desired to reduce an overlay of complex optical patterns with an accuracy finer than a micron.
Among the several objects of the present invention it may be noted the provision of an alignment scheme for a microlithographic system which provides high accuracy; the provision of such an aligning method which permits corrective information to be rapidly generated; the provision of such a method of which permits baseline correction to be determined substantially independently of other possible variables; the provision of such a method may be implemented with existing equipment; and the provision of such a method which is relatively simple and inexpensive. Other objects and features will be in part apparent and in part pointed out hereinafter.